For example, platinum or gold ores can contain more metal than that recovered in conventional wet chemical or fire-assay methods. It is thought that where there are noble metal absorbing materials derived from the ore some of the noble metal taken into solution becomes adsorbed onto these materials and is not detected. In the case of gold leached into solution by aqua regia in wet assays or by cyanide in the cyanide extraction process, the gold complex becomes absorbed by such materials and is thus undetected by solution assay. Conventionally, gold leached by the cyanide process, usually at pulp densities of 35 to 50%, may be recovered from the leach solution in a subsequent stage by contacting the solution or pulp with activated carbon, usually in a concentration range of 10 to 20 grams of carbon per litre of solution carbon in pulp (CIP) process!, but on occasions up to 40 grams per litre have been used. In some instances the carbon has been added to the leaching circuit as well (CIL process), in the same concentration ranges in order to improve gold leach rates so that the gold recorded equated with the assayed grade of the ore.
The conventional assay technique for gold is either by the wet method, which is leaching with aqua regia followed by measurement of the dissolved gold by atomic absorption spectroscopy or similar techniques, or by fire assay. In some instances when the recovery of gold by the CIP process was not up to the assayed grade, adoption of the CIL process, with addition of carbon to the leach circuit resulted in increased recovery. The amount of carbon was increased, in some cases to 40 grams per litre, until the head grade recovery was achieved. In other instances the carbon in leach (CIL) process was adopted to improve gold leach rates and gold recovery rates and thereby decrease the required number of carbon contacting tanks, thus decreasing the capital cost of construction of the gold recovery plant. In some circumstances, however, increasing the carbon loadings to the leach vessel, or the subsequent contacting stages, was found to be undesirable because of the formation of fine carbon particles caused by attrition during pulp agitation. The consequent loss of carbon with its attached gold reduced the effectiveness of the process. However, previously it was not suspected that there were also undetected values of metal in some ore samples. These comments also apply to ore concentrate and tailings.
Attempts to increase gold recovery by increasing the solubility of gold by using chelating agents have been previously proposed. There has also been a disclosure in related U.S. Pat. Nos. 5,147,617 and 5,147,618 of adding a chelating agent to solubilise insoluble salts which would block the pores of the ore, prior to leaching with SO.sub.2 in recovering the gold by an anion exchange resin in the presence of oxygen. In this unusual leaching process the addition of the chelating agent increased gold yields by 5 to 10%.
It is an object of this invention to improve recovery rates of noble metals including gold.